insect

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

insect, invertebrate animal of the class Insecta of the phylum Arthropoda. Like other arthropods, an insect has a hard outer covering, or exoskeleton, a segmented body, and jointed legs. Adult insects typically have wings and are the only flying invertebrates.

The body of the typical adult insect is divided into three distinct parts, the head, thorax, and abdomen. The head bears three pairs of mouthparts, one pair of compound eyes, three simple eyes (ocelli), and one pair of jointed sensory antennae. The thorax is divided into three segments, each with a pair of jointed legs, and bears two pairs of wings. The abdomen has posterior appendages associated with reproduction. The exoskeleton is composed of a horny substance called chitin.

Insects breathe through a complex network of air tubes (tracheae) that open to the outside through a series of small valved apertures (spiracles) along the sides of the body. In chewing insects the digestive system includes a muscular gizzard that is lacking in sucking insects. The simple circulatory system is composed of a tubular heart that pumps blood forward into the head, from which it diffuses through the tissues and back into the heart. The aquatic larvae of many insects breathe by means of external gills; some very primitive species breathe directly through the body wall.

Insect Species

There are about 900,000 known insect species, three times as many as all other animal species together, and thousands of new ones are described each year. They are commonly grouped in 27 to 32 orders, depending upon the classification used. The largest order is that of the beetles (Coleoptera). Next, in order of size, are the moths and butterflies (Lepidoptera); the wasps, ants, and bees (Hymenoptera); and the flies and mosquitoes (Diptera). Other major orders are the true bugs (Hemiptera); the cicadas, aphids, and scale insects (Homoptera); the grasshoppers and crickets (Orthoptera); the cockroaches and termites (Blattodea); and the mantids (Mantodea).

Insects are found throughout the world except near the poles and pervade every habitat except the sea (although there is one marine species of water strider). Fossil records indicate that many species exist today in much the same form as they did 200 million years ago. Their enormous biological success is attributed to their small size, their high reproductive rate, and the remarkable adaptive abilities of the group as a whole, shown by the enormous variety in body structure and way of life. The mouthparts may be adapted to chewing, sucking, piercing, or lapping and the legs for walking, running, jumping, burrowing, or swimming. Insects may feed on plants or decaying matter or prey upon other small animals (especially other insects) or parasitize larger ones; they may be omnivorous or highly specialized in their diets. They display a remarkable variety of adaptive shapes and colors that may serve either as camouflage or as warning (see mimicry). Some have stinging spines or hairs and blistering or noxious secretions, used for defense.

Reproduction

A few species, notably the fireflies, produce light, used as a signal in courtship, by a chemical reaction. The sexes are separate in insects, and reproduction is usually sexual, although in many insect groups eggs sometimes develop without fertilization by sperm (see parthenogenesis). In some insects, such as bees, unfertilized eggs become males and fertilized eggs females. In others, such as aphids, all-female generations are produced by parthenogenesis. Eggs are usually laid in a sheltered place; in a few insects they are retained and hatched internally. After hatching, the insect must molt periodically as it grows, since the rigid exoskeleton does not allow much expansion. A new, soft exoskeleton forms beneath the old one, and after each molt the insect undergoes a rapid expansion before its new covering hardens. The stages between molts are called instars; the final instar is the adult.

Metamorphosis

In nearly all insects growth involves a metamorphosis, that is, a transformation in form and in way of life. Complete, or indirect, metamorphosis is characteristic of over 80% of all insect species and has four stages: egg, larva, pupa, and adult. The wingless, wormlike larva (in many species called a grub or a caterpillar) is completely unlike the adult, and its chief activities are eating and growing. Only the simple eyes are present, and the mouth is the chewing type, even in species whose adults have other kinds of mouthparts. After several molts the larva enters a quiescent stage called the pupa; the pupa does not eat and usually does not move, but within the exoskeleton a major transformation occurs that involves the reorganization of organ systems as well as the development of such adult external structures as wings and compound eyes. In some insects the pupa is enclosed in a protective case, called the cocoon, built by the larva just before pupation. When the transformation is complete the final molt occurs: the adult emerges, its wings fill with blood and expand, and the new exoskeleton hardens. The chief function of the adult is propagation; in some species it does not eat.

Incomplete, or gradual, metamorphosis is seen in members of less advanced orders (such as locusts and their relatives and the true bugs). The larva, often called a nymph (or, if aquatic, a naiad) is usually similar in form to the adult, but lacks wings. The wings begin as external bumps on the larva, and the adult emerges from the last molt without having undergone a pupal stage.

In a few very primitive, wingless insects (such as the silverfish) there is no metamorphosis. The insect emerges from the egg as a miniature adult and the only futher changes are in size and in maturation of the reproductive organs.

Many insects are valuable as predators on the harmful species, and some are important as scavengers and as aerators of the soil (see scarab beetle). Most important, many plants depend on insects as agents of pollination; in fact, flowering plants and insects evolved together. Insects are the source of useful products such as honey, beeswax, silk, lac, and cochineal. They are a major source of food for many animals, and some are eaten by humans in many parts of the world. The fruit fly has been the major experimental animal used in genetics.

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Insect

Biology
COPYRIGHT 2002 The Gale Group Inc.

Insect

Insects are a class of arthropods . Like other arthropods, they have exoskeletons made from the carbohydrate chitin , segmented bodies, and jointed appendages . Insects are distinguished by having three major body segments (head, thorax, and abdomen), with three pairs of legs attached to the thorax. Ancestral head appendages have been modified to form antennae and mouth parts, while abdominal appendages are either absent or modified to aid in reproduction. Most insects possess wings as adults, also attached to the thorax.

Sensory Systems

The insect head bears a single pair of compound eyes, composed of many individual units, called ommatidia, each of which senses a small portion of the visual field. Hunting insects such as the dragonfly may have thousands of ommatidia per eye, while others, such as ants, have many fewer. A single pair of antennae serves as chemical sensors to help find food or mates. In many species, including the tobacco hornworm moth, the female releases airborne chemicals called pheromones that attract the male. The highly branched antennae of the male moth can detect the molecules of the female pheromone, and can track the scent to find the female over very long
distances. Chemoreceptors are also located on the feet, allowing an insect to taste its food as it walks across a leaf or a table. The numerous hairs covering the insect body are linked to mechanoreceptors, which aid its sense of touch. Some mechanoreceptors can sense changes in air pressure, useful for flying or evading a swooping predator. Receptors for carbon dioxide, water, and temperature also exist.

Ingestion, Digestion, and Excretion

Insect mouth parts vary tremendously in their shapes, reflecting adaptations to a wide variety of feeding habits. Mosquitoes, for instance, have a long hypodermic needlelike stylet, perfect for piercing skin to suck blood. Butterflies and moths, among others, have a very long, flexible strawlike mouth part, the proboscis, which they unfold to sip nectar from the base of flowers. Houseflies have a spongy tonguelike labrum for sopping up a variety of foods. Grasshoppers and beetles have small, sharp mouth parts adapted for chewing. The insect gut is divided into three regions, with most digestion occurring in the midgut. Suspended into the midgut are the Malpighian tubules, which filter nitrogenous waste from the blood and deposit it as crystals
within the gut, avoiding the water loss that urine formation would entail. In termites, the hindgut houses a complex group of protists and bacteria that digest wood.

Legs and Wings

Insect legs are used for walking and climbing. In some predatory species such as the praying mantis, the front pair of legs has been modified for capturing prey, with barbed surfaces that hold other insects tightly. Almost all insects have wings, although a few primitive forms do not. In the ants, only the reproductive members of the colony have wings, which they shed after their "nuptial flight," in which they mate with members of the opposite sex.

Respiration and Circulation

Insects do not have lungs, but instead employ a highly branched network of internal tubes, called tracheae, to deliver oxygen to the tissues. Tracheae connect with the atmosphere through openings in the exoskeleton called spiracles. Insect circulatory systems transport nutrients and wastes in a fluid called hemolymph, which is pumped into and out of internal chambers surrounding the organs, an arrangement called an open circulatory system.

Reproduction and Development

Most insects reproduce sexually, although the aphids are a notable exception. Aphids reproduce by parthenogenesis, in which the egg develops into a new organism without fertilization . In honey bees and some other social insects, only one female per colony reproduces, and males are haploid , whereas females are diploid , a system called haplodiploidy. The queen produces new (diploid) females (workers, soldiers, and future queens) from fertilized eggs. Males are produced from eggs that are not fertilized, and thus males are haploid.

Insects vary in their degree of metamorphosis during development. Butterflies, beetles, and flies, for example, undergo complete metamorphosis, in which the egg hatches into a feeding larva, which then pupates. Within the pupa, the larval tissues dissolve and rearrange into the adult form. In contrast, grasshoppers, cockroaches, and cicadas undergo incomplete metamorphosis, emerging from the egg as a miniature adult, but minus the wings and genitals. To grow, all insects must molt, or shed their exoskeleton, which then reforms around the larger individual.

Metamorphosis often allows juvenile and adult individuals of the same species to avoid competition for food. Larval moths feed voraciously and can be significant agricultural pests, while adult moths either don't feed or consume only nectar.

Diversity

Insects are the most diverse of all groups of organisms, with over 800,000 species named and many thousands, probably millions, yet to be discovered. Insect diversity may be linked to their close association with the angiosperms (flowering plants). The Coleoptera (beetles) are the most diverse of all insect orders, with at least 350,000 species, representing one fourth of all known animal species. (Asked what could be inferred about the work of the
Creator from a study of His works, British scientist J. B. S. Haldane is reported to have quipped, "an inordinate fondness for beetles.") The evolutionary reasons for the mind-boggling diversity of this single order are not clear. Other major orders of insects include the Diptera (flies), Hymenoptera (bees and wasps), Hemiptera (true bugs), and Lepidoptera (moths and butterflies). Note that each name describes the wing (ptera means "wing"). For instance, Diptera means "two wings," referring to the presence of only one wing pair in this order. In the Coleoptera ("sheath wings"), the first pair of wings is modified into a hard covering for the rear pair, which is easily observed in a lady beetle, for instance.

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Insects

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Insects

Insects are invertebrates in the class Insecta, which contains 28 living orders. The animals that make up this class have a number of distinctive features. Their adult bodies are typically divided into three parts, known as the head, thorax, and abdomen. In addition, they have three pairs of segmented legs attached to the thorax and one pair of antennae. Members of the subclass Pterygota have two pairs of wings as adults. By contrast, some relatively primitive members of the subclass Apterygota are wingless.

Taxonomists (scientists who classify organisms) have recognized more than one million species of insects, more than any other group of organisms. In addition, scientists believe that tens of millions of species of insects remain undiscovered. Currently, scientists estimate that as many as 30 million species of insects inhabit Earth; most of these are thought to be beetles. In fact, all of the insect orders are poorly known. Most of the undiscovered species of insects occur in tropical rain forests, especially in the upper parts of the forest known as the canopy.

Globally, the insects exploit a remarkable diversity of habitats. They are ecologically important as herbivores (plant-eaters), predators (meateaters who hunt their prey), parasites (who feed on living organisms), and scavengers (who feed on dead organisms). As a result of these attributes, insects are considered to be one of the most successful group of organisms on Earth, if not the most successful.

True Bugs

To a biologist, the term bug has a very special meaning. It does not refer to just any insect, as it may when nonscientists use the term. True bugs are members of the order Hemiptera. The order consists of about 35,000 widely different species. Examples of terrestrial (land-living) bugs living in North America are lice, aphids, bedbugs, stink bugs, plant and leaf bugs, assassin bugs, ambush bugs, seed bugs, lace bugs, and squash bugs. Examples of aquatic bugs are water boatmen, backswimmers, giant water bugs, water scorpions, and water striders.

Bugs have two sets of wings although in some species, the wings are greatly reduced in size and the animals cannot fly. The mouthparts of bugs are adapted for piercing and sucking. Most bugs use these mouthparts to feed on plant juices. A few are parasites of vertebrates, living on the animal's surface and feeding on its blood.

Most bugs have long, segmented antennae. They tend to have well-developed compound eyes, although some species have several simple eyes as well. Many species of bugs have glands that give off a strongly scented, distasteful odor when the insect is disturbed. The common name of one species, the stink bug, is evidence for this fact.

Some species of true bugs are brightly and boldly colored. In most cases, these bugs feed on plants that contain poisonous chemicals that also occur in the bugs. These chemicals cause the insects to taste bad, providing protection for them from predators. The bright coloring provides a warning to predators that their prey not only look beautiful, but also taste bad.

While many bugs attack agricultural crops and cause economic harm to humans, a few are health hazards also. Bed bugs, for example, feed by sucking the blood of birds or mammals, including humans. Although their bites are irritating, they do not carry disease. By contrast, the Central and South American bugs sometimes known as kissing bugs are known to transmit the parasitic protozoan that causes Chagas' disease. Chagas' disease is characterized by recurring fever and may cause serious damage to the heart muscles.

Life cycles

Insects have a complex life cycle that consists of a series of intricate transformations called metamorphoses. At each stage of its life cycle, an insect is likely to have very different body shapes, functions, and behaviors. The most complicated life cycles have four stages: egg, larva, pupa, and adult. Examples of insect orders with this life cycle include butterflies, moths, and true flies. Other orders of insects have a less complex development with only three stages: egg, nymph, and adult. Insect orders with this life cycle include the relatively primitive springtails and true bugs.

Most insects are nonsocial. However, some species have developed remarkably complex social behaviors, with large groups of closely related individuals living together and caring for the eggs and young of the group. In such groups, the young are usually the offspring of a single female, known as the queen. This social system is most common in bees, wasps, ants, and termites.

Insects and humans

A few species of insects are useful to humans. For example, we obtain honey from bees and silk from silk worms. Some insects, however, are detrimental because they transmit human diseases. For example, malaria, yellow fever, sleeping sickness, and certain types of encephalitis are caused by microorganisms. These microorganisms are transmitted by certain species of biting flies, especially mosquitoes. When one of these insects bites a human, it may ingest a disease-causing microorganism in the blood it drinks. When the insect bites a second person, it may then transfer that microorganism—along with the disease it causes—to its second victim.

Other insects eat the leaves off trees and thereby cause substantial damage to commercial timber stands and to shade trees. Insects may also defoliate (remove the leaves from) agricultural plants, or they may feed on unharvested or stored grains, thus causing great economic losses. Some insects, particularly termites, cause enormous damage to wood, literally eating buildings constructed of that material. Pesticides—chemicals that are toxic to insects—are sometimes used to control the populations of insects that are regarded as major pests.

[See alsoAgrochemicals; Butterflies; Cockroaches ]

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insect

insect Any of more than a million species of small, invertebrate animals of the class Insecta, including the beetle, bug, butterfly, ant and bee. There are more species of insects than all other species combined. Adult insects have three pairs of jointed legs, usually two pairs of wings, and a segmented body with a horny outer covering or exoskeleton. The head has three pairs of mouthparts, a pair of compound eyes, three pairs of simple eyes, and a pair of antennae. Most insects can detect a wide range of sounds through ultra-sensitive hairs on various parts of their bodies. Some can ‘sing’ or make sounds by rubbing together parts of their bodies. Most insects are plant-eaters, many being serious farm and garden pests. Some prey on small animals, especially other insects, and a few are scavengers. There are two main kinds of mouthparts – chewing and sucking. Reproduction is usually sexual. Most insects go through four distinct life stages, in which complete metamorphosis is said to take place. The stages are ovum (egg), larva (caterpillar or grub), pupa (chrysalis), and adult (imago). Young grasshoppers and some other insects, called nymphs, resemble wingless miniatures of their parents. The nymphs develop during a series of moults (incomplete metamorphosis). Silverfish and a few other primitive, wingless insects do not undergo metamorphosis. Phylum Arthropoda See also arthropod

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Insecta

Insecta (Hexapoda, insects; phylum Arthropoda, subphylum Atelocerata) Class of arthropods that have three pairs of legs and, usually, two pairs of wings borne on the thorax. Typically, there is a single pair of antennae and one pair of compound eyes. Gas exchange takes place through a system of tracheae and the gonoducts open at the posterior end of the body. The oldest fossil insects occur in Devonian rocks, and the first winged representatives are known from Carboniferous rocks. Dragonflies and beetles were established before the end of the Palaeozoic; social varieties such as ants and wasps are present in Cretaceous sediments. The evolution of the flowering plants had a marked influence on insect development, so that many new forms appeared in the Cretaceous and Tertiary Periods. More than 750 000 extant species of insects have been described. This is larger than the number of species belonging to all other animal classes combined.

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Insecta

Insecta (Hexapoda, insects; phylum Arthropoda) Class of arthropods that have three pairs of legs and, usually, two pairs of wings borne on the thorax. Typically, there is a single pair of antennae and one pair of compound eyes. Gas exchange takes place through a tracheal system and the gonoducts open at the posterior end of the body. The oldest fossil insects occur in Devonian rocks, and the first winged representatives are known from Carboniferous rocks. Dragonflies and beetles were established before the end of the Palaeozoic; social varieties such as ants and wasps are present in Cretaceous sediments. The evolution of the flowering plants had a marked influence on insect development, so that many new forms appeared in the Cretaceous and Tertiary Period. About 950 000 extant species of insects have been described. This is larger than the number of species belonging to all other animal classes combined.

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Insecta

Insecta(insects) A class of Arthropoda, members of which have three pairs of legs and usually two pairs of wings borne on the thorax. Typically there is a single pair of antennae and one pair of compound eyes. The oldest fossil insects occur in Devonian rocks and the first winged representatives are known from Carboniferous rocks. The evolution of the flowering plants had a marked influence on insect development and many new forms appeared in the Cretaceous and Tertiary Periods. More than 750 000 extant species of insects have been described. This is larger than the number of species belonging to all other animal classes combined.

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insect

insect (in-sekt) n. a member of a large group of mainly land-dwelling arthropods. Insects of medical importance include various bloodsucking insects transmitting tropical diseases; lice, whose bites can cause intense irritation and bacterial infection; and flies, which transmit organisms causing diarrhoea and dysentery to food. See also myiasis.

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In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.